Biological processes have been used for centuries, for instance, in the production of beer and wine. Recently, single cell protein processes have become a field of significant research among the biological processes. Whereas broadly speaking biological processes include all operations involving reactions between living materials and non-living materials, in the present specification and claims a more limited definition of a biological process is used. Here a biological process is intended to refer to processes involving microorganism fermentation in a fluid environment. Such microorganisms may be, e.g. bacteria or yeasts, and such fluid environments include foam environments.
One well known biological process to which this invention is applicable is a fermentation process for the production of single cell protein. A presently preferred example for such a process is described in U.S. Pat. Nos. 3,642,578 and 3,982,998. Generally, in a single cell protein fermentation process an aerobic fermentation involving a microorganism and a nutrient fluid is carried out in the presence of free oxygen supplied for instance by the injection of air. In a fermenter generally the nutrient fluid together with the microorganism are subjected to gas injection. A foam is formed in the upper portion of the fermenter whereas the lower portion of the fermenter generally contains a liquid. The foam formed is broken in a foam breaker and from this foam breaker gas is removed whereas the fluid remains in the fermenter.
From the bottom of the fermenter a microorganism containing fluid is usually withdrawn, subjected to a solid/liquid separation step, e.g. in a wash centrifuge and the recovered washed microorganism mass is thereafter dried to obtain the final product. The fluid removed during such a solid/liquid separation step contains still valuable ingredients and is therefore generally sterilized and thereafter returned to the fermenter. In the prior art procedures the sterilization of this recycle liquid is necessary to avoid any contamination of the recycled stream. The fluid introduced into the fermenter has to be absolutely sterile in most biological processes, since the smallest contamination with living organisms in the fermenter can destroy the entire reaction and result in undesired products, and a plant shut down and thus increased costs. Therefore, it has been proposed in the art to sterilize all recycled streams. This practice is today followed throughout the industry.